1. Field of the Invention
The present invention relates to an electrostatic filtering device, more particularly to an electrostatic filtering device for collecting and removing carbon particles from lubricating oil for a diesel engine.
2. Description of the Prior Art
Diesel engines have recently come into increased use for automobiles due to their lower fuel consumption compared to gasoline engines. Diesel engines, however, discharge more so-called "smoke" than gasoline engines. Much of the smoke consists of unburnt carbon particles. These particles are mostly discharged into the air with the exhaust gases, but also enter into the lubricating oil. As a result, the lubricating oil used for diesel engines becomes contaminated sooner than that used for a gasoline engine. Carbon particles dispersed in lubricating oil have a very small grain size of about 0.3 to 0.4 .mu.m (micrometer), but their great amounts accelerate the friction at sliding portions of the engine, thus shortening the lifetime of the oil.
In actual operation of automobiles with gasoline engines, the lubricating oil should be replaced approximately every 8000 km. In automobiles with diesel engines, however, the lubricating oil should be replaced approximately every 4000 km. Furthermore, it has been found that, at the time of replacement while the carbon particle concentration in the lubricating oil used in a gasoline engine is about 0.05% to 0.08% based on the weight of the oil, the carbon particle concentration in the lubricating oil used in a diesel engine is about 2% to 3%. This is in spite of the earlier replacement of the lubricating oil used in a diesel engine and reflects the severe pollution of the same.
Various approaches have been taken for removing the carbon particles from the lubricating oil. One of these approaches calls for the use of a "roll tissue paper filter" primarily composed of cotton linter pulp paper wound up in the form of a roll. The particulate pollutants adhere to the cellulose fiber surface of the roll and are subsequently removed therefrom. The efficiency of removal of carbon particles from oil by this type of filter, however, is 20% to 30%, as described later. Another approach calls for the use of electrostatic force and electrodes to attract and catch pollutants in oil. This type of oil cleaning device is highly efficient for the removal of particles such as metal, but is not so efficient for carbon particles. Still another approach calls for the use of a filter medium having small pores of about 0.2 to 0.3 .mu.m(micrometer). While this approach enables fair removal of carbon particles, the carbon particles accumulate onto the filter surface, clogging the filter and preventing the oil from effectively passing therethrough.